A motor generator functioning selectively as an electric motor and a power generator is mounted on a hybrid vehicle or an electric automobile. Such a motor generator includes, for example, a cylindrical rotor fixed to an output shaft supported rotatably around an axis, a motor stator having an inner peripheral surface away from an outer periphery surface of the rotor with a predetermined space provided therebetween, and a motor housing accommodating the motor stator. A motor stator constituting such a motor generator generally includes a core member and a coil. To insulate the core member and the coil from each other, the coil is wound around a motor bobbin, and the bobbin around which the coil is wound is fitted to the core member. Conventionally, such a motor bobbin is generally made of a synthetic resin, and, for example, polyphenylene sulfide or the like is used in a case of a concentrated winding stator (see, for example, Japanese Patent Application Publication Nos. 2005-102454 and 2002-142399). In addition, a method for integrally molding polyphenylene sulfide and a core member is disclosed (Japanese Patent Application Publication No. Hei 11-341714).
When, however, the bobbin is made of such a synthetic resin, the lower limit of the thickness of the bobbin is said to be about 0.6 mm, and the recent trends of motor generators and the like toward further increase in efficiency, increase in power output, and reduction in size are not necessarily sufficiently coped with. In addition, in the case of a large-sized motor generator used for a hybrid vehicle, an electric automobile, or the like, the method for integrally molding a synthetic resin and a core member has, for example, such a problem that the synthetic resin can hardly follow the expansion of the core member due to the change in surrounding temperature during the molding, so that cracks tend to be formed.
For the reduction in size under such circumstances, a motor bobbin has been proposed which comprises a bobbin body portion made of insulating paper and flange portions made of the insulating paper, wherein at least surfaces of the bobbin body portion and the flange portions made of the insulating paper to be in contact with a coil are made of an aromatic polyamide fiber (Japanese Patent Application Publication No. 2008-263704). In this technique, insulating paper sheets each having a structure in which heat-resistant sheets made of the aromatic polyamide fiber are provided on both of the top and bottom sides of a substrate with an adhesive agent interposed therebetween, and the adhesive agent used is an acrylic adhesive agent or a thermosetting resin composition containing an epoxy resin component, a phenolic resin component, or an acrylic resin component, and an imidazole-based curing agent component.
Here, a bobbin for a motor generator which is required to have a high efficiency and a large output has to fulfill the following five characteristics simultaneously:
1) being thin (small thickness);
2) preventing ground fault of a coil and a core member (high withstand voltage, partial discharge resistance);
3) withstanding heat generated by the coil (heat resistance);
4) having mechanical strength; and
5) having good and close contact with the core member to efficiently transmit heat generated by the coil to the core member (heat dissipation properties). In particular, the small thickness is considered to be extremely important in the sense that the thinner the bobbin is, the more the coil can be increased, so that the coil space factor can be increased, and the increase in power output can be achieved.